This invention relates to an exposure apparatus and, more particularly, to a projection exposure apparatus having an ultraviolet light source and a catadioptric projection optical system.
Exposure apparatuses for lithographically printing a pattern of a mask on a wafer are required to provide a higher resolving power. To this end, projection exposure apparatuses having a light source of short wavelength light such as a KrF excimer laser of a wavelength 248 nm, for example, are developed. Further, an ArF excimer laser (193 nm) or F2 laser (157 nm) are used to shorten the wavelength more.
Generally, projection optical systems in projection exposure apparatuses use a dioptric optical system or a catadioptric optical system. However, with light of a short wavelength, usable glass materials are limited and, for this reason, correction of chromatic aberration becomes difficult. On the other hand, a catadioptric type optical system has a large advantage because it is effective with respect to removal of chromatic aberration. There are a few types using a catadioptric optical system. For example, Japanese Laid-Open Patent Application, Laid-Open No. 300973/1994 shows one using a cube type beam splitter. Further, Japanese Laid-Open Patent Application, Laid-Open No. 10431/1998 and corresponding U.S. Pat. No. 5,537,260 show one which forms an intermediate image.
FIG. 6 shows a catadiaoptric reduction optical system such as disclosed in Japanese Laid-open Patent Application, Laid-Open No. 300973/1994, having a large numerical aperature and being used for semiconductor photography production. Light from a reticle surface 100 goes via a first lens group, a deflecting mirror 20, a second lens group, a beam splitter cube 30, and a quarter waveplate 32, in this order, and it is reflected by a concave mirror 34. The thus reflected light passes again the beam splitter cube 30 and, after being transmitted through a third lens group, it is collected on a wafer surface 50. For further reduction of higher-order aberrations, the concave mirror 34 has an aspherical surface.
FIG. 7 shows a catadioptric optical system such as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 10431/1998, which is a small-size catadioptric system having a sufficiently large numerical aperture in the image side and work distance, and having a resolution of a quarter-micron unit in the ultraviolet region. Light from an object is reflected by a concave reflection mirror CM of a first imaging optical system S1 and, thereafter, it forms an intermediate image on the light path of the first imaging optical system S1. This intermediate image is then imaged by a second imaging optical system S2 upon the wafer surface, through a first optical path changing member M1. The first imaging optical system S1 has an imaging magnification which is to be set from 0.75 to 0.95, by which the light path deflection by the first light path deflecting member M1 is enabled and, on the other hand, the image side numerical aperture NA of the optical system is made large. Further, the value of L1/LM (L1 is the axial distance between the object plane and the intersection of the optical axes of the first imaging optical system S1 and of the second imaging optical system S2, and LM is the axial distance between the object plane and the concave mirror CM) is set to be from 0.13 to 0.35, by which the image side working distance of the optical system is assured and, additionally, coma and distortion aberration are well corrected.
FIG. 8 shows another example of a catadioptric type projection optical system such as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 20195/1998 and corresponding U.S. Pat. No. 5,835,275.
However, where an exposure light source such as an ArF excimer laser or F2 laser is used, the absorption of light by glass materials of the projection optical system becomes very large, and therefore, the amount of thermal aberration produced during the exposure process becomes large. Further, the aberration is variable with a change in atmospheric pressure or with illumination conditions.
In order to correct this, a lens or lenses close to the object plane may be moved. However, it needs to take a large change of aberration to the lens motion and, therefore, if the movement distance is large, the mechanical positional precision of the moved lens is degraded. In summary, it is difficult to reduce the aberration to be corrected.
Further, where the numerical aperture (NA) is small, the remaining amount of symmetrical aberrations such as curvature of field and astigmatism, and spherical aberration after correcting the magnification and distortion are small. However, since the remaining amount of symmetrical aberrations such as curvature of field and astigmatism, and spherical aberration increase in proportion to the fourth power or square of the NA, they become a large value not negligible, with a large NA. The NA is becoming large with further miniaturization of a semiconductor device. Thus, it is one of the most important issues to decrease symmetrical aberration.
It is accordingly an object of the present invention to provide an exposure apparatus by which symmetrical aberrations can be reduced.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: a projection optical system of catadioptric type; and an optical element disposed on a reciprocating light path of said projection optical system, said optical element being movable to adjust a symmetrical aberration of said projection optical system.
In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: a projection optical system of catadioptric type; and an optical element disposed on a reciprocating light path of said projection optical system, said optical element being movable to correct at least one of spherical aberration, astigmatism, and curvature of field.
In these aspects of the present invention, at least one of spherical aberration, astigmatism and curvature of field, produced with heat, pressure or an illumination condition, may be corrected.
The light projected on said optical element may be reflected by a mirror, disposed at an aperture stop position or a position equivalent thereto, and may be directed again to said optical element.
The mirror may comprise a concave mirror or a convex mirror.
The optical element may be disposed adjacent to said mirror.
An ArF laser may be used as an exposure light source, or an F2 laser may be used as an exposure light source.
Coma aberration and distortion aberration as asymmetrical aberrations of said projection optical system may be adjusted by motion of an optical element disposed on a single way of the light path.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer to a device pattern by use of an exposure apparatus as recited above; and developing the exposed wafer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.